Circular scanner having superimposed dither



NOV. 5, 1968 M, F. COHEN ET AL CIRCULAR SCANNER HAVING SUPERIMPOSEDDITHER 5 Sheets-Sheet l Filed Feb.

CEA/fie 0f SCI/V NOV. 5, 1968 M F. COHEN ET Al. 3,409,777

CIRCULAR SCANNER HAVING SUPERIMPOSED DITHER Filed Feb. 1966 5Sheets-Sheet 2 Ir-E'. 3.

NGV. 5, 1968 M, F COHEN ET AL 3,409,777

CIRCULAR SCANNER HAVING SUPERIMPOSED DITHER Filed Feb. 1966 5Sheets-Sheet 5 Iz-n.-

FT-.5a.

NOV. 5, 1968 M, F, COHEN ET AL 3,409,777

CIRCULAR SCANNER HAVING SUPERIMPOSED DITHER Filed Feb. 3, 1966 5Sheets-Sheet 4 M. F. COHEN ET AL 5 Sheets-Sheet 5 Nov. 5, 1968 CIRCULARSCANNER HAVING SUPERIMPOSED DlTRRR Filed Feb. e, 196e United StatesPatent O Fice 3,409,777 CIRCULAR SCANNER HAVING SUPERIMPOSED DITHERMurray F. Cohen, Roslyn Heights, Irvin S. Englander, Jackson Heights,and Sheldon Girsch, Bayside, N.Y., assignors to Kollsman InstrumentCorporation, Elmhurst, N.Y., a corporation of New York Filed Feb. 8,1966, Ser. No. 525,927 9 Claims. (Cl. Z50- 203) ABSTRACT F THEDISCLOSURE A tracking device using an image dissector tube for trackingaround an illuminated disk, such as a planet, using a circular scanningpattern having a superimposed dither thereon. The tracking deviceoperates in a spiral acquistion mode to acquire the planet or body to betracked, and after acquisition, is automatically placed in a tine trackmode for pointing an axis toward the center of the body being tracked.Radial sizing circuits are provided to adjust the effective scan circlediameter, with the scanning process operable even though an image is ina crescent phase.

This invention relates to a novel tracker device, and more particularlyrelates to a novel tracking device for tracking a planet or otherequivalent illuminated disk or disk portion whereby the tracking devicecan determine the coordinates of the center of the illuminated disk orportion thereof.

In accordance with the present invention, the tracking device is soarranged as to observe the outline of the illuminated disk through themeans of a circular scan having superimposed dither. That is to say, acircular scan is applied which generally follows the outline of theillaminated disk being tracked where a superimposed sinusoidalmodulation is applied to the circular scanning path so that the scandirection continually oscillates into and out of the periphery of thedisk being tracked. This modulation shall be hereinafter referred to asdither.

When using the novel dither concept, a novel system is further used inconnection therewith so that a best circular scan t will be obtained forthe outline of an illuminated disk such as a lunar or planet image as itappears on the cathode face of an image dissector tube. For a nearlycircular image; for example, the appearance of the full moon at a largedistance, the displacement of the sean circle and the image results inan error signal at the fundamental circular scanning frequency. For anoblate planet such as Jupiter which appears as an elliptical disk in thefocal plane, a null condition can be obtained which corresponds to thecenter of the two foci of the ellipse.

In the case of the nearly circular planet image, -copending applicationSer. No. 158,300, filed Dec, 1l, 1961, in the name of Jacob S.Zuckerbraun, entitled Photosensitive Horizon Scanner for Space Vehicle,and assigned to the assignee of the present invention (now U.S. Patent3,246,160), provides a planet tracking technique whereby at least threepoints are located on the periphery of the disk with the center andradius of the planet being determined by computational methods.

It has been found that with this technique the presence of variousanomalies such as dark spots on or near the image edge necessitate thesampling of many points for greater weighted accuracy.

In accordance with the invention, a circular scan pattern with a highfrequency radially directed dither permits the sampling of many pointsduring a single cycle around the planet edge. Appropriate signalprocessing is then provided to handle the various sample data points3,409,777 Patented Nov. 5, 1968 in each cycle, and an analog centeringtechnique is then used to determine the planet center rather than thedigital computation required in the aforementioned copending applicationSer. No. 158,300.

lt should be noted that the planet tracker of above noted copendingapplication Ser. No. 158,300 is not a true edge tracker, since a trueedge tracker will follow the shape of the edge regardless of the edgecontour. Thus, if the planet were in its crescent phase, the edgetracker would outline the exact shape of the crescent. If dark spotswere present in the planet rim, the edge tracker would move inside theplanet disk to nd the bright edge.

In accordance with the present invention, the novel tracking techniqueusing a scan pattern with a circular constraint can be made to find thebest circular match to the planet edge.

Many star trackers and planet trackers have been further developed inthe past which use various tracking techniques. Thus, it is known that astar may be tracked by interrupting or oscillating the star image withrespect to a defining aperture. Such an arrangement is shown incopending application Ser. No. 244,665, tiled Dec. 14, 1962, in the nameof Burt Walker, entitled Star Tracker Scanning System Using a CircularScanning Pattern and a Square Aperture, -and assigned to the assignee ofthe instant invention now U.S: Patent 3,244,896. In this arrangement,signals are developed by a detector, and the star is centered byappropriate processing methods. Thus, the tracking center willcorrespond to the center of illumination as defined by the centralmaximum of the image pattern. Clearly, at great distances, a planet willappear as a small object in the focal plane of this type of star trackeroptics. By increasing the F number of the optics, the planet can bemagnified whereby its geometrical properties are recognizable. Theplanet can still be tracked by star tracking techniques of the abovenoted type if tracking to the center of illumination proves sufticient.

However, since it is normally the geometrical center of the planet whichis desired, obtaining the center of illumination may result insignificant errors caused by the planet phases and variations in surfaceretiectance or emissivity over the illuminated planet portion.

In accordance with the present invention, the novel tracker structurewill be provided with the ability to sample the planets rim at a largenumber of points around the circle of the illuminated disk so thatvarious anomalies in the sampling operation can be integrated out. Thisarrangement is obtained through the use of a circular scan which hassuperimposed thereon a novel dither variation.

A high resolution optical system of novel construction is furtherprovided which will greatly magnify the image to provide an extendedimage in a very small field of view. This, in combination with a highsignal-to-noise ratio which is obtainable in the electronic portion ofthe apparatus, contribute directly to tracking accuracy. In addition,the novel system of the invention eliminates all mechanical moving partsand provides an ability to obtain a directional reading even though theobject being tracked is ot the optical axis of the system.

Accordingly, a primary object of this invention is to provide a noveltechnique for tracking illuminated disks or portions thereof.

Another object of this invention is to provide a novel high accuracyplanet tracker.

A still further object of this invention is to provide a novel highaccuracy planet tracker which has no moving parts.

Another object of this invention is to provide a novel tracking mode forradiant energy tracking devices in which the device to be tracked ispresented as an illuminated disk or portion thereof which is circularlyscanned in an optical scanning pattern which has a high frequency dithersuperimposed thereon.

These and other objects of this invention will become apparent from thefollowing description when taken in connection with the drawings, inwhich:

FIGURE la schematically illustrates a typical image dissector incross-sectional view.

FIGURE lb illustrates the spiral image acquiring mode of scanningoperation used in the image dissector of FIG- URE la during theacquisition mode of operation.

FIGURE 1c illustrates the fine tracking mode of operation in which acircular scanning pattern having a superimposed dither thereon ismatched to the illuminated planet image.

FIGURE 2 illustrates the scan geometry or the displacement of the scancircle center from the planet center during the tine tracking mode ofoperation.

FIGURE 3 schematically illustrates a pulse width demodulator which isused to determine the misalignment between the scanning pattern and theplanet image being tracked.

FIGURES 4a, 4b and 4c are to be taken in conjunction with FIGURES 5a, 5band 5c, respectively, to illustrate the manner in which the planet imagemay be sized to the scan radius.

FIGURES 6a and 6b illustrate the manner in which a light terminator on aplanet crescent is determined.

FIGURES 7a through 7d illustrates the tracking con- 9 ditions from fullillumination of the planet to its quarter phase.

FIGURE 8 is a functional block diagram of the electronic controlcircuitry used in accordance with the invention.

FIGURE 9 illustrates the novel double Cassegrain objective used inaccordance with the invention.

FIGURE 10 shows a novel modification of the Cassegrain objective ofFIGURE 9 in which the lens surfaces are merged and the optical operationis improved.

While the concepts of the present invention may be carried out withvarious types of scanning mechanisms, it shall be described herein withreference to an image dissector arrangement. Image dissectors are wellknown to the art, and the image dissector as used in accordance with theinvention is schematically illustrated in FIG- URE la.

Thus, in FIGURE la, a suitable optical system having an optical axis 20awhich is directed toward the object to be tracked by a suitable servoingsystem (not shown) is located in front of the photocathode 21 of animage dissector tube 22. The image dissector tube 22 will includesuitable detiection plates or coils, schematically illustrated as plates23 and 24, for altering the position of the electron beam emitted fromthe rear of photocathode 21 and toward the image defining aperture 25(which may be anodic) in orthogonal directions. That is to say, in theschematic illustration of FIGURE 1a, plate 23 may be deemed to causevertical positioning of the electron beam, while plate 24 will causehorizontal positioning of the beam. Image dissectors of this type arewell known to the art and the function and construction of plates 23 and24 need no further description.

In addition to these control plates, suitable accelerating electrodeswill be provided for accelerating the electron beam toward aperture 25,and similarly suitable focusing electrode means will be provided forfocusing the beam toward a focal plane which is contained in the planeof opening 25.

A suitable electron detection means such as a photomultiplier 26 havingoutput electrodes 27 and 28 is thcn located behind aperture 25, therebyto receive an electron stream when the focusing and control electrodesof the image dissector cause the electron beam issuing from photocathode21 to pass through aperture 25.

In describing the acquisition pattern of scanning and the trackingpattern of scanning in FIGURES lb and 1c. reference may be made to theanalogy of oscillation of the aperture 25. It should be noted, however,that there is no physical movement of aperture 25, but that differentpoints of the photocathode 21 will be observed passing through aperture25 by suitably controlling the position of the electron beam issuingfrom any particular point of photocathode 21 through the control ofcontrol electrodes 23 and 24.

Thus, in the acquisition mode of operation, as illustrated in FIGURE 1b,suitable control potentials are applied to deflection plates 23 and 24so that control conditions make it possible to spirally sample areas ofphotocathode 21 so that when a point is reached which contains an imagein the photocathode 21, the electron beam issuing therefrom shall passthrough aperture 25.

In a similar manner, and in FIGURE 1c, a circular scanning pattern isshown, which has a dither superimposed thereon, which scans an image 31formed on photocathode 21 by the objective where the scanning line 30 isagain a sampling scanning of the electron image issuing fromphotocathode 21 by suitable control of the deflection electrodes 23 and24.

A cquisilion mode In virtually all types of tracking devices, it isfirst necessary to acquire the signal or object which is to be tracked.A novel acquisition mode of operation is provided in the presentinvention which includes the use of a spiral scan pattern. Thus, in theacquisition mode, the optics 20 will focus the planetary disk somewhereon the cathode 21 of the image dissector tube 22 if it is in the totaltield of view of the apparatus. An electron stream will then issue fromthe photocathode at the point where the image is received.

In the search mode, the control electrodes 23 and 24 will cause thiselectron beam to move in a spiral pattern, as shown in FIGURE lb, wherethis spiral pattern is independent of the specific location onphotocathode 21 from which the electron stream emanates. This spiralaction will, in effect, be scanned by aperture 25 so that if the imageappears, for example, in shaded area of FIGURE lb, when the scanningpattern reaches this position in the spiral, the electron beam will passthrough aperture 25 and be sensed by photo-multiplier 26.

In the search mode, this scan is in the form of an inwardly directedspiral, as illustrated by the arrows in FIGURE lb, with a signal pulsegenerated by photomultiplier 26 being compared to the sweep signalsapplied to electrodes 23 and 24, thereby the determine the relativeposition of image 40 on the photocathode 21 so that a preliminarydetermination may be made of the planet coordinates with respect to theoptical axis 20a of the telescope used for the tracker. Suitable D-Ccurrent slgnals may then be applied to the deflection plates 23 and 24(or deection coils, if coils are used) which will drive the spiral scancenter to a position somewhere within the planetary disk.

The equipment may then be automatically placed in its fine track mode ofoperation to point the optical axis 20a directly at the effective centerof the disk 40.

Fine track mode of operation In the fine track mode of operation, asillustrated in FIGURE 1c, the scanning mode is provided by a 200 cycleper second circular scan which has superimposed thereon a 10 kilocyclesinusoidal modulation to result in the dither scanning pattern 30 ofFIGURE lc. Moreover, the radius of the circular scan will coincide withthe radius of the target image 31 in FIGURE lc.

As will be described more fully hereinafter, when the respective centersof the scan circle 30 and the planet image 31 are aligned and the planetand scan radii are matched, the dither scan will be equally divided intime between being out of the scan image and into the scan image (theduty cycle of the effective scanning aperture will be evenly dividedbetween the illuminated and dark regions of space). Where alignmentexists, the output of the image dissector photo-multiplier 26, after theuse of appropriate shaping circuitry, will be a series of square pulsesof equal duration. However, if the respective centers of the scan circleand planet image are displaced, a pulse width modulated rectangular wavetrain will be developed with a period identical to that of the 200 cycleper second scan circle, but with a pulse width modulation correspondingto the displacement between the centers of circles and 31 in FIGURE 1c.

FIGURE 2 illustrates the geometry of the scanning conditions when thecenter of the scanning pattern 30 is displaced from the center of theilluminated disk 31 by a distance d in the negative y direction. Thevariable I describes the instantaneous distance of the scan circle fromthe planet edge as a function of the phase angle 0.

The variable l which is the instantaneous displacement of the center ofoscillation from the planetary rim is derived from triangle OPO asfollows:

where R is the radius of the planet disk and scan circle for' matchedconditions, while a equals some function of 0.

of this sinusoid will depend on the orientation of the planet withrelation to the scan center. When the orientation of the planetdisplacement changes by some particular angle, the phase of the sinusoidwill shift accordingly. Thus, the position of the planet can bedetermined by phase detection of the fundamental harmonic component ofthe pulse width modulated scan signals.

One technique for demodulating the pulse width modulated wave prior to asynchronous phase detection process is illustrated in FIGURE 3. FIGURE 3illustrates an input terminal 40 to which the pulse width modulatedtrain is applied to parallel connected constant current sources 41 and42 which have additive series connected polarities. The output of theparallel connected constant current sources 41 and 42 are then connectedto a common integrator 43 which has an output terminal 44 which willpresent a sinusoidal wave train whose relative phase will be dependentupon the angle 0 in FIGURE 2.

Thus, in FIGURE 3, the signal pulse train at input 40 will swing betweenground and some fixed negative voltage so that the signal will swingequally in the positive f(t) K[Ti (on) -Tt' (06)] Radial sizing It willbe apparent that operation in the fine scan mode 6 requires the matchingof the circular scan radius 30 to the planet image radius 31 in FIGURES1c and 2.

In accordance with the invention, the nature of the novel scan pattern30 permits the generation of an error signal proportional to a sizedifference in the radial sizes of the planet image radius 31 and thescan radius 30. Moreover, the polarity of this error signal willindicate whether the scan radius is smaller or larger than the planetimage radius. Furthermore, these error signals do not depend onmaintaining alignment between the planetary and scan centers.

FIGURES 4a, 4b and 4c illustrate the geometry of the fine track modewith the scan center and planet image centers aligned in each case andwith ,matching radii (FIGURE 4a) with the radius of the scanning patternsmaller than the planetary image (FIGURE 4b), and with the radius of thescanning pattern greater than the radius of the planetary image (FIGURE4c).

If, as in FIGURE 4a, the planet and scan radii are equal, then as theaperture is oscillated in and around the planet disk, the dwell timespent within the disk will be equal to the dwell time spent outside thedisk.

The generated waveform in this case will be that shown in FIGURE 5awhich plots output voltage from the photo-multiplier 26 as a function oftime where the generated waveform will be seen to be a square wave forthe condition of FIGURE 4a.

Where the scan radius differs from the planet radius, however, as inFIGURES 4b and 4c, the waveform becomes unsymmetrical, as shown inFIGURES 5b and 5c. Thus, in FIGURES 5b and 5c, it can be seen that thewaveform of FIGURE 5a changes from a longer pulse duration (FIGURE Sb)to a shorter pulse duration (FIG- URE 5c) when the scan radius issmaller and larger, respectively, than the planetary image.

It can be easily shown by a Fourier analysis that an asymmetricalrectangular waveform contains the second harmonic of the fundamentalfrequency signal. The polarity of this second harmonic will then differin the cases of FIGURES 5b and 5c, thereby to define an error sizingsignal. Note that the frequency of this second harmonic component willbe 20 kiocycles since the fundamental for this particular situationwould be the l0 kilocycle dither.

Accordingly, once the planet is acquired, means are automaticallyprovided wthen the dither mode of scanning becomes operative toappropriately size the scanning radius to the planetary image.

Operation when the planetary image is in a crescent phase In theforegoing, it has been presumed that a full disk is available in thescanning operation. It will often occur, however, that the planet to betracked will be in one of its various illumination phases as determinedby the solar and line-of-sight angle changes with respect to the planetso that problems could be created as compared to automatic trackingofthe planet vertical.

It should be further noted that the planet tracker may operate inregions other than the visible light regions and could also operate inthe ultraviolet or near infrared portion of the spectrum. Even then,however, such problems could occur as due to uneven planet heating andthe like, so that it becomes necessary to provide means for tracking,even though the planet is in any of its various phases including thecrescent phase.

As previously described, the determination of the center of illuminationof a disk-shaped object could result in large position uncertainties dueto surface albedo variations and phase variations. However, the novelscanning apparatus of the present invention can be used for allillumination phases at a suicient distance from the planet so that atleast a sc micircular outer edge of the planet will be presented throughthe employment of suitable logic and signal processing circuitry.

Before discussing this circuitry, it is useful to understand thegeometric properties of the planet light terminator as described inFIGURES 6a and 6b which illustrate a projected geometric technique fordetermining the shape of the planet terminator. l

Assuming that sunlight shining on the planet is in the direction shownby the labeled arrows in FIGURE 6b, a terminator 51 will be definedwhich marks the boundary between light and dark on the planet 50. Thisterminator 51 can, of course, be observed from various angles by thescanning system depending upon the relative position of the vehiclecarrying the scanner with respect to the planet 50.

FIGURE 6a shows a projected ortho-normal grid which can be constru; edby the projection lines extending from FIGURE 6b to FIGURE 6a, and thenconnecting the intersected grid lines to generate an ellipse whichdescribes the planet terminator. That is to say, vertical lines 52, 53,54 and 55 taken from FIGURE 6b, are drawn in FIGURE 6a as circles 52a,53a, 54a and 55a. Horizontal lines 52h, 53h, 5412 and 55b are thenprojec.cd from the intersections of lines 52 through 55 with terminator51 to intersect respective circles 52a through 55u with theseintersecting points defining the terminator ellipse 56 in FIGURE 6a.

The semi-major axis of ellipse 56 will always correspond to the radiusof the planet S0. while the eccentricity of the ellipse will vary fromone at quarter phase to zero at fu l phase. The actual terminator of theplanet will depart from an ellipse due to planet oblateness and surfaceanomalies. but for purposes of the present invention. it is suflicientto presume that the terminator will be elliptical.

When using the novel rotating dither scan of FIGURE lc, signals will beavailable where the dither amplitude is sufficiently large to penetratethe illuminated portion of the planet image.

Thus. in FIGURES 7a through 7d which show various tracking conditions,in the condiLions of FIGURES 7a and 7b, signals from the dither pattern30 will be obtained for a full cycle (in the condition of FIGURE 7b, thedither pattern always enters the terminator line) while in theconditions of FIGURES 7c and 7d, dither signals -are obtained for onlyapproximately I/2 cycle. However,

in each of conditions 7a through 7d, the center of the planet can betracked without ambiguity since the signals obtained are obtained fromcircular portions of the planet.

Note that in the condition of FIGURE 7b, there is no relative posiion ofa scan circle and planet where all members of the pulse train will havethe same width.

However, a tracking null position can be obtained when error signals inopposite quadrants will balance out. Thus,-

the null track posiion for the condilion of FIGURE 7b may be biasedoff-center and to the right in favor of the elliptical portion of thedisk. Since one-half of the disk is always circular, however, adiscrimination technique may be used to track only the circular portion.

More particularly, and where the condition of FIGURE 7b is obtained, thesignal delivering information from thc photo-multiplier 26 of FIGURE lacan be turned on for 1/2 of its. duty cycle. This duty cycle may becontrolled by suitable switching means which is initially phased at 6:0with reference to the scanning circle.

When tracking the circular side of a planet, the waveform obtained for asmall angular displacement of the planet center and scan center can beshown to be of the form of l/z sine wave. The harmonic content of the V2sine wave is well known, and is as follows:

Thusl when tracking non-circular portions, the harmonic content of thepulse width demodulated wave will depart from the 1/2 sine wave and oddharmonics will appear since the terminator in FIGURE '7/1 is ellipticalCit in shape. Therefore, it becomes possible for the circuitry todistinguish between the tracking of the planet edge and the ellipticalterminator in the condition of FIG- URE 7b.

Functional description of complete operating system The completeelectronics and other circuitry used in the control and processing ofthe output signals of the photo-multiplier 26 of FIGURE la are describedin FIGURE 8 in detail. It will be noted, however, that for purposes ofsimplicity, the various components are illustrated in block diagram formwith all of the block components being of types well known to theirvarious respective arts and which could be easily and readilyconstructed by those skilled in the various arts.

Refering now to FIGURE 8, the circuitry associated with the novelscanning system includes suitable power sources, which are not shown,along with a 1 cycle astable multivibrator 101, a 400 astablemultivibrator 102 and a 2 kilocycle astable multivibrator 103. The 1cycle multivibrator 101 is connected to a fiip-tiop 104g which, in turn,drives a sawtooth generator 105 which is connected to an acquisitiondisconnect switching means 106. The acquisition disconnect switchingmeans 106 is then connected to a variable gain amplifier which is asawtooth modulator 107. The 400 cycle astable multivibrator 102 isconnected through a frequency divider 108 to a 200 cycle filter andamplifier 109, which, in turn, is connected to a gain contol input ofthe variable gain amplifier 107. The 2 kilocycle astable multivibrator103 is then connected to a 10 kilocycle filter and amplifier 104. Thevariable gain amplifier 107 is then connected to a vertical ringmodulator 110 and a 90 phase shifter 111. The output of 10 kilocycleamplifier 104 is connected to the control electrode of vertical ringmodulator 110 with the output of vertical ring modulator 110 andamplifier 107 connected to the adder 112. The adder 112 is thenconnected to a vertical defiection amplifier 113 which is subsequentlyconnected to the vertical defiection electrode of the image dissector ofFIGURE la such as electrode 23.

The 90 phase shifter 111 is then connected along with the output of 10kilocycle amplifier 104 to the horizontal ring modulator 114 and theoutput of horizontal ring modulator 114 and 90 phase shifter 111 arethen connected to adder 115 which is, in turn, connected to thehorizontal defiection amplifier 116 which is connected to the horizontaldeection electrode of the image dissector of FIGURE la such as electrode24.

The output of the photo-multiplier 26 in FIGURE la is schematicallyillustrated in the upper left-hand corner of FIGURE 8 as connected to awide band amplifier 120. The output of amplifier 120 is connected to thenarrow band amplifier 121 which amplifies about the second harmonicoutput signal of amplifier 120. Amplifier 121 is then connected tosynchronous demodulator 122 which is, in turn, connected through the lowpass filter 123 to variable gain amplifier 107, thereby to generate theradial sizing signal.

Synchronous demodulator 122 is, in turn. connected to frequency doubler124 which receives a 10 kilocycle signal from the l0 kilocycle filterand amplifier 125 which is connected to the 10 kilocycle adder 126. The10 kilocycle adder 126 then has applied thereto the signals from theoutputs of vertical deection amplifier 113 and horizontal deflectionamplifier 116. These output signals are further connected to synchronousdemodulators 127 and 128, respectively, which are each connected to theimage dissector output signal from amplifier 120 taken through the pulsewidth demodulator 129 which may be of the type shown in FIGURE 3.

Each of synchronous demodulators 127 and 128 are further connected tolow pass filters 130 and 131, respectively, to generate output signalswhich are functionally related to the coordinates of the center of theplanet being tracked and could, for example, be connected to a servosystem to keep the optical axis of the tracker pointed toward theircenter position.

The amplified image tube output signal from amplifier 120 is furtherconnected to the monostable multivibrator 140 and to one input terminalof a compare gate 141. The output of monostable multivibrator 140 isconnected to the other terminal of compare gate 141 with the output ofcompare gate 141 connected to the variable current source 142 and to theacquisition disconnect switching means 106.

In addition, the output of compare gate 141 is connected to the x pointstorage means 143 and the y point storage means 144 which are eachconnected to the 200 cycle sampling signal and proportional D-C signaloutputs of vertical deflection amplifier 113 and horizontal defiectionamplifier 116, respectively.

In the following description of operation of FIGURE 8, it will be moreconvenient to regard the aperture of the image dissector tube of FIGURE1a as being the element which scans over the image plane of thephotocathode 21 instead of the actual scanning of the entire electronimage analog of the optical image. Thus, where reference is madehereinafter to spiralling or oscillating apertures, it is to beunderstood that the electron image analog of the planet is the movingelement with the aperture 2S remaining fixed.

The initial scan pattern in the operation of the device is the spiralscan pattern (FIGURE 1b) which searches for a target in the field ofview of the photocathode 21 of FIGURE la. The spiral scan mode isgenerated by the 1 cycle astable multivibrator 101, and the asymmetricfiip-fiop 104a which actuates the sawtooth generator 105. The sawtoothwaveform output of generator 105 serves as a linear gain control for thevariable gain amplifier 107 which also receives a sine wave controlsignal at 200 cycles per second from the input branch including blocks102, 108 and 109.

The output of the variable gain amplifier 107 will then be a sawtoothmodulated sine wave which is connected directly to the verticaldefiection amplifier 113 and horizontal defiection amplifier 116 with anappropriate 90 phase shift obtained by the phase shifter 111. These twosignals will then produce the spiral scan mode, as schematicallyillustrated in FIGURE 1b.

In order to obtain the rotating `dither pattern, a 10 kc. signal issuperimposed upon a 200 cycle waveform. Thus, the output of 10 kc.amplifier 104 is fed directly to the vertical ring modulator andhorizontal ring modulator 114 to appear directly at the output ofamplifiers 113 and 116 independently of the phase shift required for thespiral acquisition mode of operation.

The reason for modulating the 10 kilocycle signal in the manner shown inFIGURE 8 can be understood from the following:

The resultant waveform in the fine track mode is to be a circle whoseradius matches the planet disk radius. A dither pattern is superimposedon this circle whose arnplitude is constant around the circle and whoseamplitude vector is always pointed radially toward the center of thecircle. The large circular pattern is the resultant of the amplitudevector of two sine waves whose phases are displaced by 90. Similarly,the superimposed dither pattern is also the vector resultant of two sinewaves which, as pointed out above, are in phase.

Thus, at the axis points on the scan reference circle, a maximum valueof x must be accompanied by a minimum value of y. The reverse is true atthe iy axis points. Similarly, at the 1r/4, 31r/4 and 31r/2 positions,the x and y values must both be equal to 1/\/2 in order to maintain anequal amplitude and radial direction.

The reason why the dither pattern is also superimposed on the spiralscan mode is simply that the presence of fine oscillations during thespiral acquisition will in no way degrade the search capability of thescanner and it eliminates the need for switching the pattern in at alater time.

During the spiral scan mode and as soon as the aperture crosses theplanetary disk image (using the analog of a moving aperture) a pulse isgenerated and will pass through the amplifier 120. This pulse is thenfed to a pulse width discriminating circuit comprising the monostablemultivibrator 140 and compare gate 141 which determines a minimumacceptable pulse width. The function of the pulse width discriminator isto determine a position on the planet disk which is near the planetdiameter. When the planet pulse width is greater than the pulse width ofthe monostable multivibrator 140, recognition signals are sent to the xand y points storage 143 and 144, respectively, whereby D-C bias currentvalues corresponding to the instantaneous amplitudes of the x and ydefiection plate output signals are fed back to the x and y defiectionelectrodes to drive the scan oscillation center to a point inside theplanet disk.

At the same instant of recognition, the spiral scan generator isdisabled by the acquisition disconnect switch 106 and the scan circle ismade to collapse to a point and then slowly spiral outward by connectingthe variable current source 142 into the sawtooth modulator 107 whichacts assentially as a variable gain amplifier whose gain control isregulated by current inputs.

The slowly expanding circle continues until the fine track mode isinitiated by means of a 10 kilocycle presence signal which is obtainedin the output of amplier 120, and is generated by the dither crossingthe edge of the planet disk.

The complete system now goes into the fine tracking mode, and performsthree basic functions; tracking in the x and y or vertical andhorizontal directions, radial sizing, and sampling of a circular segmentof the planet image.

The l0 kilocycle signals from the image dissector output amplifier 120are fed into the pulse width demodulator 129 and the tracking signalsare synchronously demodulated in the x and y or vertical and horizontaldirections by multiplication with the reference signals which are tappedfrom the deflection electrodes and filtered for 200 cycles.

Analytically, synchronous demodulation is described as follows:

The A-C components are filtered in the low pass filters 130 and 131 sothat the tracking signals are D-C signals whose polarity will bedependent on the phase of the incoming signal.

Radial sizing is accomplished by synchronously demodulating the secondharmonic of the l0 kilocycle rectangular wave. That is, the 10 kilocyclecomponent of the scan is tapped off both defiection electrodes andconnected to the 10 kilocycle adder 126 where signal addition willresult in a waveform of constant amplitude. The output of adder 126 isthen applied to the l0 kilocycle amplifier and filter and is doubled inthe frequency doubler 124 to provide a reference signal for thesynchronous demodulation of the 20 kilocycle component of the signal inthe synchronous demodulator 122. As in the tracking signal case, the A-Ccomponent is filtered out, and the polarity of the D-C signal is used toobtain correct radial sizing by feeding back the error signal to thevariable gain amplifier 107 through the low pass filter 123.

Optical system An important consideration for the best operation of thenovel tracker of the present invention lies in a suitable optical systemwhich can provide a long focal length in a small package. This isespecially true where the tracker is to utilize an image dissector asthe detecting element. In this case, and for commercially availableimage dissectors, the eld -will be a circle 0.7 inch in diameter, and inorder to preserve tracking accuracy, a long focal length is necessary.

This becomes extremely difficult since, in the ordinary photographictelephoto lens, the so-called telephoto effect is only 0.75 to 0.85where the telephoto effect is the ratio of the overall length of thelens from front to focal plane to the focal length of the lens.

If a eld of view of 30 minutes is used, the focal length of theobjective would become 80 inches and a telephoto effect of 0.75 wouldresult in a system 60 inches long.

It is common practice to use a Cassegrain type of objective where thetelephoto effect is to be reduced where Cassegrain systems typicallywill give a telephoto effect of approximately 0.4 so that the overalllength of the system would be reduced to about 32 inches.

In order to shorten the Cassegrain objective still further, the power ofthe primary mirror must be increased. This. however, has the effect ofincreasing the convergence of the rays reected by the primary so thatthe secondary moves closer to the primary with its power alsoincreasing. When this procedure is carried too far, it results in anunbalanced system which will have low definition over the entire fieldof view.

In accordance with the present invention, a novel Cassegrain system isprovided, as illustrated in FIGURE 9, wherein a second Cassegrain systemis interposed in the light path of the primary Cassegrain system.

Thus, in FIGURE 9, a primary Cassegrain system is formed of a concavemirror 200 which directs light toward a convex mirror 201. The concavemirror 200 has an opening 202 therein, and normally the light fromconvex mirror 201 would focus in a plane on the righthand side ofopening 202.

In accordance with the invention, however, a second Cassegrain systemwhich includes concave mirror 203 and convex mirror 204 is interposed inthe primary Cassegrain system with the light from convex mirror 204passing through aligned opening 205 in mirror 203 and opening 202 inmirror 200 toward a focal plane which includes the photocathode of theimage dissector tube.

This novel arrangement reduces the telephoto effect of the Cassegrainobjective to approximately 0.15 without.

' however, unduly affecting the definition over the field of view.

A further modication of the novel double Cassegrain of FIGURE 9 isillustrated in FIGURE 10 which reduces the number of optical surfacesrequired in an arrangement of the type shown in FIGURE 9. Thus, inFIGURE 10, the Cassegrain system is provided with the usual priy marymirror 200 and secondary mirror 201 of FIGURE 9 where, however, thefunction of the tertiary and quaternary mirrors 203 and 204 of FIGURE 9are merged into the primary and secondary mirrors 200 and 201,respectively. The resulting system is extremely compact and results in atelephoto effect of approximately 0.11. Note that the primary andsecondary mirrors 200 and 201 in FIGURE 10 are both made to be asphericin order to correct spherical aberration and coma. The resilientastigmatism is so small as to be completely negligible in FIGURE 10.

A negative lens 210 is then placed just in front of the focal plane ofmirror 201 to fiatten the field at the photocathode of image dissector229.

Although this invention has been described with respect to its preferredembodiments, it should be understood that many variations andmodifications will now be obvious to those skilled in the art, and it ispreferred, therefore, that the scope of the invention be limited not bythe specific disclosure herein, but only by the appended claims.

The embodiments of the invention in which an exclusive privilege orpropertly is claimed are defined as follows:

1. An illuminated body tracking device comprising telescope means forforming an image of said illuminated body; image dissector means havinga photocathode, control means for controlling a beam of photo-electronsemitted from said photocathode and output circuit means for receivingsaid beam of photo-electrons and generating output signals in responsethereto; control circuit means connected to said control means forselectively directing photo-electrons from selected regions of saidphotocathode toward said output circuit means to effect scanning of saidphotocathode; said control circuit means generating a circular scan ofsaid photocathode having a superimposed dither thereon.

2. The device of claim 1 wherein said control circuit means includes arelatively low frequency voltage source connected to said contro]electrodes for generating a relatively Iow frequency circular scanningfrequency and a relatively high frequency voltage source forsuperimposing a relatively high frequency dither on said relatively lowfrequency circular scan.

3. The device substantially as set forth in claim 2 wherein said controlcircuit means further includes means responsive to the second harmoniccomponent of the output of said output circuit means connected to saidcontrol means for adjusting the radius of said circular scan to theradius of the image of said illuminated body on said photocathode.

4. The device as set forth in claim 3 wherein said circular scan has afrequency of about 200 cycles per second and said dither has a circularscan of about 10,000 cycles per second.

5. The device as set forth in claim 3 wherein said control meansincludes acquisition circuit means for causing a spiral scanning patternwhich spirally scans the full area of said photocathode whereby anoutput signal from said output circuit means signals the existence of anilluminated body in the field of view of said photocathode.

6. The method of scanning the periphery of an illuminated body against adark background; said method comprising the steps of circularly scanningaround the periphery of said illuminated body, and superimposing adither on said circular scan to cause said scanning path to continuouslycross the dark to light boundary at the periphery of said illuminatedbody.

7. A tracking device for a planet tracker; said tracking deviceincluding means for forming an image of the planet to be tracked, andscanning means for scanning said image; said scanning means for scanningsaid image including means for photoelectrically observing selecteddiscrete portions of said image and output signal generating means forgenerating an output signal when said means for photoelectricallyobserving said image detects an illuminated discrete area; said scanningmeans further including control means connected to said means forphotoelectrically observing selected discrete portions of said image toselect discrete areas of said image to be observed in a predeterminedscanning pattern; said predetermined scanning pattern comprising acircle having the diameter of said image and having a high frequencydither superimposed thereon whereby, during scanning, said means forphotoelectrically observing said image repetitively moves around andinto and out of the periphery of said image.

8. The device as set forth in claim 7 which further includes sizingcircuit means for adjusting the diameter of said scanning circle to thediameter of said image; said sizing circuit means including firstcircuit means responsive to the phase of the second harmonic of thedither frequency connected to said output signal generating means, andsecond circuit means connecting said rst circuit means to said controlmeans for adjusting the diameter of the said circular scan to thediameter of said image.

9. The device as set forth in claim 8 which further includes acquisitioncontrol circuit means connectable to said control means; saidacquisition control circuit means 13 14 driving said photoelectricobserving means in a spiral 3,246,160 4/ 1966 Zuckerbraun 250-203pattern. 3,290,505 12/1966 Stavis Z50-203 References Cited UNITED STATESPATENTS RALPH G. NILSON, Primary Examiner.

3,240,942 3/1966 Birnbaum etal Z50-203 5 M. ABRAMSON, AssislanfExaminer.

